1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting a liquid, and particularly to an ink jet head from which an ink is ejected to conduct recording on a recording medium, and a liquid ejection method.
2. Description of the Related Art
A method in which a heat generating element is used to eject an ink is widely used as a liquid ejection method for an ink jet recording apparatus. This method is such a method that thermal energy is generated by a heat generating element arranged in a flow path (nozzle) to which an ink is supplied, thereby causing film-boiling of the ink around the heat generating element to generate a bubble and applying kinetic energy to the ink by the bubbling pressure to eject the ink toward a recording medium from an ejection orifice. This method involves a problem that the heat generating element is damaged by cavitation caused by the extinction of the bubble generated on the heat generating element.
U.S. Pat. No. 7,152,951 discloses a liquid ejection head and a liquid ejection method by which damage to the heat generating element caused by the cavitation can be inhibited. In this liquid ejection head, an ejection orifice is arranged in opposition to the surface of the heat generating element with a center of the ejection orifice deviated from a center of the heat generating element toward the upstream side or the downstream side of the ink flowing direction. A bubble thereby communicates with the air at a site where the bubble is hard to be divided upon ejection of a droplet, so that the bubble is inhibited from being divided into a portion of the upstream side and a portion of the downstream side in the ink flowing direction. As a result, the bubble can be prevented from being divided to remain in a flow path, and so cavitation that generally easily occurs on the downstream side in the ink flowing direction and damage to the heat generating element attending thereon can be inhibited. This technique is particularly effective in a liquid ejection head having a heat generating element of nearly a square with an aspect ratio of about 1.
Japanese Patent Application Laid-Open No. 2008-238401 discloses such a technique that an ejection orifice and a flow path are arranged at a high density of 1,200 dpi (1,200 dots per inch (2.54 cm)) or more from a demand for further densification of ink jet recording. Specifically, in Japanese Patent Application Laid-Open No. 2008-238401, plural ejection orifices and flow paths are arranged in a row at a density of 1,200 dpi.
Japanese Patent Application Laid-Open No. H04-10940 and Japanese Patent Application Laid-Open No. H04-10941 disclose an example of a method of ejecting an ink in an ink jet recording apparatus.
When ejection orifices and flow paths are arranged at a high density of 1,200 dpi or more as disclosed in Japanese Patent Application Laid-Open No. 2008-238401 to attempt to eject a droplet of 1.5 pl or more, the flow path needs to be formed slenderly. Accordingly, a (slender) heat generating element having a large aspect ratio according to the form of the flow path needs to be used unlike the invention described in U.S. Pat. No. 7,152,951. Specifically, the aspect ratio of the heat generating element needs to be controlled to 2.5 or more (a vertical length is 2.5 times or more as much as a horizontal length). As a result, damage to the heat generating element by such cavitation as illustrated in FIG. 12 of U.S. Pat. No. 7,152,951 may occur.